1. Field of the Invention
The present invention relates to a communication system, a mobile terminal, a relay apparatus, and a transfer route learning method.
2. Description of the Related Art
In the Internet according to internet protocol (IP) (for example, refer to J. Postel, “Internet Protocol,” Request for Comments 791, September, 1981), a router splits a network into plural subnets. In the same subnet, a data frame is transferred according to layer 2 protocol represented by Ethernet (registered trademark) (for example, refer to “LAN/MAN CSMA/CD Access Method,” IEEE 802.3, March, 2002). A network in the same subnet is composed of a router preset in a boundary between the subnets, a layer 2 switch or a hub which houses plural hosts and transfers the data frame between the router and the hosts, and the hosts which connect to the layer 2 switch or the hub and make communication with an arbitrary party on the other end. The transfer of the data frame in the subnet is performed by using a layer 2 address. As a typical layer 2 address, there is a MAC address (for example, refer to “LAN/MAN CSMA/CD Access Method,” IEEE 802.3, March, 2002). In the case of transmitting a packet addressed to a host present in the subnet, the router refers to a destination IP address of the packet, and obtains a MAC address of the destination host by using neighbor discovery protocol (for example, refer to T. Narten, et al., “Neighbor Discovery for IP version 6 (IPv6),” Request for Comments 2461, December, 1998). After resolution of the destination MAC address, the router creates a data frame, in which the MAC address of the destination host is set as a destination address of the layer 2, and a MAC address of the router is set as a transmission source. Then, the router transfers the data frame into the subnet. As shown in FIG. 1, upon receiving the data frame from an arbitrary port, the hub copies the data frame, and transmits the data frame from all the ports excluding the port, which has received the data frame. Hence, the hub consumes a band in the subnet excessively, and sometimes, collision of data occurs in the hub.
Meanwhile, as shown in FIG. 2, the layer 2 switch owns a bridge forwarding table (BFT) in which mapping of the MAC addresses and the transfer destination ports are registered. Upon receiving the data frame from the arbitrary port, the layer 2 switch refers to the destination MAC address of the data frame, and searches whether the destination MAC address is registered in the BFT. When the destination MAC address is registered in the BFT, the layer 2 switch transmits the data frame from the ports registered in the BFT. Hence, the layer 2 switch can utilize the band in the subnet efficiently as compared with the hub, and prevents the data collision. Upon receiving the data frame from the arbitrary port, the layer 2 switch registers, in the BFT, the transmission source MAC address of the received data frame, and the port, which has received the data frame. When the transmission source MAC address is not registered in the BFT, the layer 2 switch newly registers the transmission source MAC address in the BFT. When the port, which has received the data frame, is different from a port registered in the BFT though the transmission source address is already registered therein, the layer 2 switch changes the port registered in the BFT to the port, which has received the data frame. When the port registered in the BFT coincides with the port, which has received the data frame, the layer 2 switch updates a value called an aging time, which represents a valid period of registration information. By the above-described method, the layer 2 switch creates and updates the BFT.
In IPv4, the destination address resolutions in the layer 2 uses address resolution protocol (ARP) (for example, refer to D. C. Plummer, “Ethernet Address Resolution Protocol,” Request for Comments 826, November, 1982). The ARP transmits, to a broadcast address, a request message describing destination IP addresses for which the address resolution is desired to be performed. Hence, the ARP has had a problem of consuming the band in the subnet excessively. The neighbor discovery protocol that is layer 2 address resolution protocol of IPv6 transfers, to a solicited-node multicast address, a neighbor solicitation message requesting the address resolution, thus making it possible to utilize the band in the subnet efficiently. However, unless the layer 2 address registers a multicast address (for example, a MAC multicast address) of the layer 2 in the BFT, the layer 2 switch undesirably transmits the data frame addressed to the MAC multicast address from all the ports. Accordingly, traffic made by the layer 2 switch becomes substantially equivalent to that of broadcast. As a method of allowing the layer 2 switch to learn the MAC multicast address in order to solve the above-described problem, there is MLD snooping (for example, refer to M. Christensen, “Considerations for IGMP and MLD Snooping Switches,” draft-left-magma-snoop-11.txt, May, 2004). The MLD browses an ICMP multicast listener report message (for example, refer to R. Vida, “Multicast Listener Discovery Version2 (MLDv2) for IPv6,” Request for Comments 3810, June, 2004) transmitted by the host, and registers the MAC multicast address in the BFT. The ICMP multicast listener report message sets the layer 2 destination address of the data frame as a MAC multicast address of a multicast group in which the host joins. Hence, as shown in FIG. 3, the layer 2 switch registers, in the BEFT, the layer 2 destination address of the ICMP multicast listener report message, and the port which has received the ICMP multicast listener report message. According to the MLD snooping, the layer 2 switch can transmit the MAC multicast address only to the port to which the host joining in the multicast group connects, and accordingly, can utilize the band efficiently.
Here, in such a case where the host moves, it is necessary to update the BFT of the layer 2 switch in response to a moving destination of the host. For example, a utilization mode is mentioned, in which, in the subnet, an access point (AP) is connected to the router or the layer 2 switch, thus making it possible for such a mobile host (MH) to connect to the Internet under a wireless environment. The AP offers a wireless link to the MH, owns a BFT equivalently to the layer 2 switch, and also has a function to transfer the data frame. When the AP to which the MH connects is switched, it is necessary for the AP and the layer 2 switch that are present in the same subnet to change the registration information in the BFT concerning the MH which has switched the AP to which the MH is to connect. IEEE 802.11 as a standard of the wireless LAN defines a method of updating the BFTs of the AP and the layer switch 2 in response to the movement of the MH as IEEE 802.11f (for example, refer to “Inter Access Point Protocol,” IEEE 802.11f, July, 2003). In the IEEE 802.11f, a layer 2 update frame is defined, and when the MH switches the AP to which the MH concerned is to connect, the frame concerned is transmitted into the same subnet. As shown in FIG. 4, the AP and the layer 2 switch which have received the layer 2 update frame register, in the BFTs, the transmission source MAC address (MAC address of the MH) of the layer 2 update frame, and the port which has received the layer 2 update frame.
However, it is only a MAC unicast address of the MH that the layer 2 update frame can allow the AP and the layer 2 switch to learn, and there has been room for improvement in updating the BFTs efficiently.
Meanwhile, the ICMP multicast listener report is usually transmitted at the time when the host is activated. Hence, the ICMP multicast listener report is not defined so as to be transmitted every time when the MH moves. Although there are some implementations adapted to transmit the ICMP multicast listener report in response to the movement of the MH, such implementations transmit the ICMP multicast report only in the case of a movement in a layer 03, that is, only in the case where the subnet is changed. Hence, the ICMP multicast listener report is not transmitted in the case where the AP is switched in the same subnet. In this case, a neighbor solicitation for the resolution of the layer 2 address transmitted on the last one hop of the MH in the case where an arbitrary party (correspondent node) on the other end of the communication in the Internet starts the communication with the MH as the destination is transmitted to false ports by the AP and the layer 2 switch, or is transmitted from all the ports thereby.
In this connection, in consideration of the above-described problems, it is an object of the present invention to provide a communication system, a mobile terminal, a relay apparatus, and a transfer route learning method, which learn a data link layer unicast address of the MH and a data link layer multicast address thereof in response to the movement of the MH, thereby updating the BFTs efficiently.